Japanese Patent Laying-Open No. 2004-357388 has disclosed a step-up/down converter of a multi-phase multiplex type that has a step-up function and a step-down function. This step-up/down converter is controlled such that currents having phases shifted from each other are passed for respective phases of the converter.
FIG. 7 is a waveform diagram for illustrating a multi-phase (three-phase) converter.
Referring to FIG. 7, for a single phase, a battery current that is not smoothed with a control period of 100 μs has a large amplitude, and a large ripple current flows even after the smoothing. Conversely, when a three-phase converter operates with the same control period, PWM signals of respective phases may have a phase difference of 120 degrees therebetween, and thereby a ripple current having a period of 33.3 μs and a reduced amplitude is passed in the smoothed battery current.
Thus, the three-phase converter causes the ripple current of a smaller amplitude than the single-phase converter.
The step-up/down converter described above is generally formed of a chopper circuit including reactors and switching elements.
Some kinds of such step-up/down converters control changing or switching of a switching period of the switching elements based on a passed power. An inverse number of the switching period is called a carrier frequency. For example, when the passed power is large, the control may take place to increase the switching period (i.e., to decrease the carrier frequency) for decreasing a switching loss.
FIG. 8 shows an example of consideration for illustrating the changing of the carrier frequency.
Referring to FIG. 8, as can be seen from port output timing indicated by triangular marks, control information including a carrier frequency and a duty ratio is provided for a converter of three phases, i.e., Y-, V- and W-phases from a central control unit to a control unit of each phase. At times t0, t9, t18, t27 and t36 indicated by the triangular marks, the settings held in the control units for the respective phases are collectively rewritten. Each control unit executes the switching control of the corresponding phase based on the setting held thereby.
In each of U-, V- and W-phase current waveforms, an turned-on period Ton of the switching element is present between a downward peak point of the waveform and a subsequent upward peak point, and a turned-ff period Toff of the switching element is present between the upward peak point of the waveform and the subsequent downward peak point. The control period is equal to (Ton+Toff).
From time t0 to time t18, a time difference (phase difference) of Ty is present between the U-phase and the V-phase, and a time difference (phase difference) of Ty is also present between the V-phase and the W-phase.
At time t18, it is assumed that an instruction that doubles the control period with the duty ratio kept at 50% is issued simultaneously to all the phase control units. However, each phase control unit cannot change the control period at some midpoint in the control period.
Accordingly, the control period of the U-phase current waveform immediately doubles at time t18, but the control period of the V-phase doubles at time t20. The control period of the W-phase doubles at time t19.
Thus, the control period of each phase changes only when the ongoing control period expires (i.e., at the downward peak point) after the instruction is received.
In the example shown in FIG. 8, the control period of the W-phase changes earlier than the V-phase. Consequently, in the current waveform at and after time t20, time difference Ty is kept between the U- and V-phase current waveforms, but a time difference TyA different from that before time t18 occurs between the U- and W-phase current waveforms. Thus, such a result occurs that the same phase difference cannot be kept between the phases at the time of carrier switching. Consequently, the output voltage ripple of the multi-phase voltage converting device may be large.